Methods consisting in determining the thermal properties (heat conductivity or heat capacity) of various materials are numerous. There are thus known so-called continuous measurement methods which have the drawback of being long to use while permitting only each time the measurement of a single value. These methods such as those which utilize a heat shielded plate or a radial flux do not permit visualizing the changes in a material such as the change of structure in the course of measurement.
Another large group of measurement methods relates to methods for measuring a transitory regime. A first so-called flash method described particularly in the article of A. Degiovanni: "Diffusivity and Flash Method", Revue Generale de Thermique, Volume XVI, N.degree. 185 of May, 1977, has the drawback of providing only a mean value for a given interval of variation of the temperature of the material. A second method using a shock probe described in the article A. Lobo/C. Cohen "Measurement of thermal conductivity of polymer melts by line-source methods", Polymer Engineering and Science--January, 1990, Volume 30, N.degree. 2, and a study of the thermal transfer coupling and preliminary physico-chemical transformation ratio, 1987, B. Garnier/D. Delaunay, has fairly poor precision (in the range 10-20%). This method is moreover fairly long to use when it is desired to make a measurement of elevated temperatures because it is necessary before the measurement to bring the conductivity meter and the materials to a uniform temperature. As a result, the methods described above are usable only with difficulty if the transformations take place in the material during the duration either of preparation or of performance of the measurement. They are therefore of little applicability in this case. Such is the case in particular with the process and device described in EP-A-0.124.104. In this patent, there is proposed a device which is generally similar in physical arrangement to the device of the invention to determine the thermal conductivity and thermal capacity of materials. The essential difference from the present invention resides in the use of a heating element disposed between two specimens which requires modifying linearly the temperature and in making the supposition that over a given range of temperature the conductivity is constant. Because of this, again, by means of such a device, account is not taken of the possible transformations of the material.
A third group of methods utilizing algorithms for identification by a reverse method permits avoiding the drawbacks mentioned above (duration of handling, impossibility of making more than one measurement, impossibility of giving results in certain regions of materials in which transformations of the material take place). Their precision however is insufficient. Thus, in certain configurations of measurement of the flows, it is necessary to know precisely the value of certain thermal contact resistances or the thermal conductivity of certain constituent materials of the conductivity meter. These supplemental measurements are not always easy to perform. Moreover, these methods often require the implantation of sensors within the specimens, thus giving rise to a certain number of problems.